Analog/digital converters (A/D converters) are electrical circuit arrangements used for the conversion of a signal, such as a voltage or current, from the analog domain to the digital domain. A variety of different A/D converter types exist.
One known A/D converter design uses a so-called sigma-delta (or delta-sigma) modulator that samples an analog input signal at a relatively high sampling rate in order to perform a noise shaping function. This oversampling is commonly performed at a multiple of the so-called Nyquist sampling rate of the input signal frequency. Thereby, quantization noise power is spread over a bandwidth equal to the sampling frequency, thereby reducing the noise density in the band of interest. Sigma-delta A/D converters typically include a loop filter in the forward signal path to push some of the quantization noise into the higher frequency spectrum beyond the band of interest and a quantizer for quantizing the output signal of the loop filter.
In particular the development of sigma-delta A/D converters in low voltage technologies faces new design challenges that may require the development of new A/D architecture concepts. Sigma-delta A/D converters with a relatively low oversampling ratio (OSR) may at least partly solve problems like clock jitter, loop delay and stability problems by using multibit quantizers. However, new CMOS processes make the implementation of flash quantizers with a large number of quantization levels relatively difficult. Another problem is associated with the fact that a sigma-delta modulator produces a higher bit rate at its output compared with the bit rate produced by a so-called Nyquist converter having an equivalent resolution which may mostly be due to an inefficient low pass decimation filtering that is often performed on the digital output of the sigma-delta modulator.